1. Field of the Invention
The present general invention concept relates to an inkjet printhead, and more particularly, to a thermal inkjet printhead that can improve print quality.
2. Description of the Related Art
Generally, inkjet printers are devices forming an image having a predetermined color on a printing medium by ejecting micro ink droplets, which are fed by inkjet printheads attached to ink cartridges, onto a desired region of the printing medium. Such inkjet printers can be classified as shuttle type inkjet printers in which inkjet printheads print by moving in a perpendicular direction to the transfer direction of a printing medium, and line printing type inkjet printers including array printheads having a size corresponding to the width of a printing medium, and which have been recently developed in order to realize high-speed printing. In the line printing type inkjet printers, a plurality of inkjet printheads are arranged on the array printheads in a predetermined pattern, and the line printing type inkjet printers print when the array printheads are fixed and as the printing medium is transferred through the printers. Thus, the line printing type inkjet printers are highly preferred since such printers can print at high speed.
Depending on the ink ejecting mechanism, inkjet printheads can be classified into two types: thermal inkjet printheads and piezoelectric inkjet printheads. In more detail, a thermal inkjet printhead generates bubbles in the ink using a heat source, and ejects ink droplets using the expansion of the bubbles. On the other hand, a piezoelectric inkjet printhead ejects ink droplets using pressure that is applied to the ink by deforming a piezoelectric material.
FIG. 1 is a partially cutaway perspective view of a conventional thermal inkjet printhead. Referring to FIG. 1, the conventional thermal inkjet printhead has a structure in which a chamber layer 20 and a nozzle layer 30 are sequentially stacked on a substrate 10. An ink feed hole 11, to feed ink, is formed in the substrate 10. An ink chamber 22 that can be filled with ink fed through the ink feed hole 11 and a restrictor 24 connecting the ink chamber 22 to the ink feed hole 11 are formed in the chamber layer 20. A nozzle 32, for ejecting ink, is formed in the nozzle layer 30. A heater 14, to heat ink filled in the ink chamber 22 to generate bubbles, is formed on the substrate 10 in the ink chamber 22. In the conventional thermal inkjet printhead having the above structure, when electric current is supplied to the heater 14, ink adjacent to the heater 14 is heated and bubbles are generated and expanded. Due to the expansion of the bubbles, the ink filled in the ink chamber 22 and the nozzle 32 is ejected from the ink chamber 22 through the nozzle 32 in the form of droplets. Thereafter, new ink is fed from the ink feed hole 11 into the ink chamber 22 through the restrictor 24.
In the conventional thermal inkjet printhead of FIG. 1, only some of the ink filled in the ink chamber 22 and the nozzle 32 is ejected from the ink chamber 22 during ink-ejection, while the ink that remains in the ink chamber 22 and the nozzle 32 stays in the heated state. The remaining heated ink is mixed with the new ink fed through the ink feed hole 11 in order to be ejected in a next operation. However, the mixed ink has a higher temperature than that of the ink initially filled in the ink chamber 22 and the nozzle 32, and is ejected from the ink chamber 22 through the nozzle 32 during the ink-ejection during the next operation. As the ink-ejection proceeds, the temperature of the ink filled in the ink chamber 22 and the nozzle 32 is increased. Accordingly, the temperature of the ejected ink is increased. Generally, the higher the temperature of the ink, the lower the viscosity of the ink, thereby further increasing the quantity of the ejected ink. Accordingly, in the conventional thermal inkjet printhead having the above structure, as a printing operation proceeds, a degradation of print quality may occur, in which the density of an image that is to be eventually printed becomes higher than that of an image initially printed.
In addition, in FIG. 1,for the conventional thermal inkjet printhead having the above structure, the temperature of the substrate 10 is increased because some of heat generated by the heater 14 continuously accumulates on the substrate 10 around the location on which the heater 14 is formed on the substrate 10 during a printing job. The heat-accumulation phenomenon may seriously occur in the recently developed thermal inkjet printhead operating at a high frequency, which has been recently developed in order to realize high-speed printing. Likewise, when the temperatures of the substrate 10 and ink are increased as a printing operation proceeds, oxygen, nitrogen, carbon dioxide or the like dissolved in the ink evaporates, and thus, air bubbles may be generated. In addition, these air bubbles generated by the heater 14 may not completely disappear, and thereby may remain in the form of minute bubbles. The air bubbles and remaining bubbles deteriorate the ejection property of ink, thereby deteriorating the print quality of an image.